Automobile scanning system

ABSTRACT

A dual energy x-ray imaging system searches a moving automobile for concealed objects. Dual energy operation is achieved by operating an x-ray source at a constant potential of 100 KV to 150 KV, and alternately switching between two beam filters. The first filter is an atomic element having a high k-edge energy, such as platinum, gold, mercury, thallium, lead, bismuth, and thorium, thereby providing a low-energy spectrum. The second filter provides a high-energy spectrum through beam hardening. The low and high energy beams passing through the automobile are received by an x-ray detector. These detected signals are processed by a digital computer to create a steel suppressed image through logarithmic subtraction. The intensity of the x-ray beam is adjusted as the reciprocal of the measured automobile speed, thereby achieving a consistent radiation level regardless of the automobile motion. Accordingly, this invention provides images of organic objects concealed within moving automobiles without the detritus effects of overlying steel and automobile movement.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.12/135,196, filed on Jun. 8, 2008, which claims the benefit of U.S.Provisional Application No. 60/943,040, filed on Jun. 9, 2007, both ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates to the x-ray imaging of automobiles to detectexplosives, hidden persons, contraband, and other security threats.

Criminals and terrorists frequently conceal security threats inautomobiles, such as explosives being transported into undergroundparking facilities for the purpose of destroying skyscrapers; illicitdrugs being smuggled across borders; and illegal immigrants beingbrought into the country. Searching the automobile by visual inspectionis time consuming and often ineffective. For instance, persons andcontraband hidden in the dashboard or within the seats cannot bevisually detected. X-ray inspection systems are now in use that candetect these security threats to some extent. These prior art systemsuse x-rays that are either transmitted through the automobile, or backscattered from the automobile, to form an image. The x-ray image isinspected by the security personnel to detect the presence of hiddensecurity threats. However, prior art systems cannot readily distinguishthe organic matter that comprises security threats from the steelforming the automobile. Further, this inability prevents the prior artsystems from employing automated threat detection software programs.Still further, prior art systems are inefficient in producingcontraband-revealing images at the ultra-low radiation doses that areacceptable for such security examination.

BRIEF SUMMARY OF THE INVENTION

The present Invention overcomes these limitations of the prior art byproviding an apparatus and method capable of acquiring dual-energytransmission x-ray images of automobiles passing through a securitycheckpoint. A linescan x-ray imaging system is provided in an archwayconfiguration, whereby the automobile being examined drives slowlythrough a fan beam of x-ray radiation, with the driver and passengersremaining safely within the vehicle. High-energy and low-energy x-rayspectra are alternately selected for the fan beam of radiation, allowingx-ray images of the automobile to be acquired at two separate x-rayenergies. The switching spectra are formed by operating the x-ray sourceat approximately 120 KV, and switching the beam filtration materialbetween 6.35 mm thick copper and 0.762 mm thick bismuth sheets, orsimilar elements. Detection of the fan beam of radiation is accomplishedwith a linear array of detectors, such as Cadmium Tungstate or CaesiumIodide crystals mounted on photodiodes. The dual-energy images areconverted to a steel-suppressed image and calibrated for measuring thethickness of organic material. The mass of each organic object in theautomobile is subsequently calculated from the calibrated image bysumming the pixel values over the projection of the object in the image.Organic masses greater than a specified threshold trigger an alert tosecurity personnel for secondary inspection.

The present Invention operates with only a few microRem of radiationexposure to the driver and passengers of the automobile. This radiationexposure is regarded as trivial under radiation protection standards andappropriate for general purpose security examination. One aspect of thepresent Invention adjusts the output intensity of the x-ray source tomatch the speed of the automobile, thereby maintaining the highest imagequality for the allowable radiation dose.

It is therefore the goal of the invention to provide an improved methodand apparatus for detecting security threats and contraband concealedwithin automobiles passing through a security check point. Another goalof the invention is to facilitate the inspection of automobiles for thepresence of explosives, hidden persons and contraband. Yet another goalis to provide a dual-energy imaging apparatus and method capable ofdetecting organic objects in the presence of overlying steel. A furthergoal is to provide a uniform radiation exposure to automobiles beingexamined regardless of the automobile's speed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall schematic depiction in accordance with the presentinvention.

FIG. 2 is a depiction in accordance with the present invention.

FIG. 3 is a depiction in accordance with the imaging geometry of thepresent invention.

FIG. 4 is a depiction in accordance with the detector of the presentinvention.

FIG. 5 is a depiction in accordance with one aspect of the presentinvention.

FIG. 6A and FIG. 6B are graphs in accordance with the present invention.

FIG. 7A and FIG. 7B are graphs in accordance with the present invention.

FIG. 8 is a graph in accordance with the present invention.

FIG. 9 is a flowchart in accordance with one aspect of the presentinvention.

FIG. 1 shows the overall general operation of the Invention. Thescanning apparatus 100 is contained within an archway 101, about 3048 mmhigh and 3048 mm wide, spanning across the roadway 201. The automobilebeing examined 202 approaches the scanning apparatus and is stopped by afirst gate-arm 204. The first gate-arm 204 is raised, allowing theautomobile 202 to slowly drive through the archway 101 until stopped bythe second gate-arm 205, at a position 203 after the archway 101. Anx-ray assembly 103 is mounted at the top of the archway 101, directing afan beam of x-rays downward to a linear array detector assembly 104,resting on the roadway 201. Automobile motion sensors 106 provide anelectronic output of the speed of the automobile 202, 203 as it passesthrough the archway 101. Support members 102 hold the archway 101upright and prevent persons on foot from entering the x-ray examinationarea. The x-ray image data generated by the scanning apparatus 100 isreceived by computer system 105 in an operator area 206. The computersystem 105 processes the data to generate an image on the computersystem display monitor, showing objects concealed within the automobile.

FIG. 2 shows a more detailed view of the archway 100 of the Invention.X-ray assembly 103 comprises an x-ray source 110 emitting x-rays 125downward to a linear slit collimator 113, resulting in a fan beam ofx-rays 114 passing to the linear array detector 104. A rotating chopperwheel 112 is affixed in the x-ray beam next to the x-ray source 110 andturned by a motor 111. The outlines of a large automobile being screened202 and a person 207 are shown for size reference.

FIG. 3 shows a more detailed description of the x-ray imaging apparatusof the Invention. The x-ray source 110 is of conventional construction,such as having a fixed anode, a 1.651 mm×1.651 mm focal spot, andoperating at about 120 KV and 2 ma. As known in the art, an x-ray tubeoperating at a selected KV results in an internal electron energy thatis numerically the same. For example, it is equivalent to state that anx-ray tube operates at 120 KV, and that it operates with an electronenergy of 120 keV. X-ray shielding in the design of x-ray source 110blocks the x-ray beam 125 from all directions except downward, where itilluminates the linear slit collimator 113. Linear slit collimator 113consists of a sheet of x-ray opaque material 121, such as lead ortantalum, with an opening 122 to permit the passage of x-rays. Theopening 122 is approximately 1066.80 mm long and 2.54 mm wide, andpositioned about 762 mm below the x-ray source 110. The fan x-ray beam114 exits the linear slit collimator 113 and propagates to the lineararray detector 104, located about 2286 mm below. The projected width ofthe fan x-ray beam 114 is therefore 2.54 mm×3048 mm/762 mm=10.16 mmwhere it strikes the active detection area 130 of the linear arraydetector 104. Focal spot blurring at this location is 1.651 mm×2286mm/762 mm=4.953 mm. Combining the projected width with the focal spotblurring results in a total width of the fan x-ray beam 114 being 10.16mm+4.953 mm=15.113 mm where it strikes the active detection area 130 ofthe linear array detector 104.

The x-ray beam 125 passes through the rotating chopper wheel 112immediately upon exiting the x-ray source 110. The rotating chopperwheel 112 consists of a copper disk approximately 355.6 mm in diameterand 6.35 mm thick. A plurality of bismuth plates 115 are affixed to therotating chopper wheel 112 at uniformly spaced angular increments. In apreferred embodiment, four to ten such bismuth plates 115 are used, witha thickness of about 0.762 mm, with the copper under each bismuth plateremoved. This results in the x-ray beam 125 passing through either 6.35mm copper or 0.752 mm bismuth at any one instant, as the rotatingchopper wheel 112 is rotated around a vertical axis 116. Rotation of therotating chopper wheel 112 is accomplished by an electric motor 111. Therate of rotation is adjusted to provide alternating exposures of aboutthree milliseconds through the copper beam filter followed by threemilliseconds through the bismuth beam filter. In a preferred embodimenthaving six bismuth plates 115 this corresponds to a rotation rate of1,250 rpm.

As thus described, the combination of the x-ray assembly 103 and thelinear slit collimator 113 forms a fan beam of x-rays 114, wherein thex-ray spectrum alternates every three milliseconds between 120 KV with6.35 mm copper filtration and 120 KV with 0.762 mm bismuth filtration.The fan beam of x-rays 114 strikes the active detection area 130 of thelinear array detector 104. The active detection area 130 is about 15.24mm wide, slightly wider than the 15.113 mm total width of the fan x-raybeam 114. In one preferred embodiment the linear array detector 104 isfolded into a “U” shape. As known in the art, such folded detectorsprovide the same operation as flat linear array detectors, but have theadvantage of being more compact. The active detection area 130 comprisesa plurality of detector elements. In one preferred embodiment, 320detector elements are used with each measuring about 15.24 mm by 15.24mm. Each detector element can be formed by one of the known detectiontechniques, such as scintillators mounted on photodiodes; scintillatorsmounted on photomultiplier tubes; or direct detection using germanium,silicon, or cadmium zinc telluride devices. In a preferred embodimenteach detector element is a 15.24 mm×15.24 mm×3.81 mm thick scintillatorcrystal mounted on a 9.906 mm×9.906 mm photodiode. The scintillationcrystal may be either CsI(Ti) or CdWO4. A white reflective paint isapplied to all exposed surfaces of the crystal to maximize lighttransfer from the scintillator to the photodiode. Readout electronics ofsuch detectors are known in the art.

The alignment of the linear slit collimator 113 is critical.Specifically, the focal spot of the x-ray source 110, the opening 122,and the active detection area 130, must remain coplanar. This alignmentproblem is aggravated by the automobile 202 being required to drive overthe linear array detector 104, which may move the detector relative tothe other assemblies. Accordingly, in one preferred embodiment a methodand apparatus is provided to automatically align the linear slitcollimator 113. Linear actuators 120 are affixed to the extreme ends ofthe linear slit collimator 113 as shown in FIG. 3. Under softwarecontrol, the first linear actuator moves the first end 121 of the linearslit collimator 113 a distance of about 12.70 mm in three seconds.During this three second period the signal from the corresponding end ofthe linear array detector 104 is recorded and stored in relation to theinstantaneous linear actuator position. Afterward, the controllingsoftware program searches the stored data for the greatest measuredsignal level, and moves the first linear actuator back to the positioncorresponding to the greatest measured signal level. This procedure isthen repeated at the opposite end of the linear slit collimator usingthe second linear actuator.

FIG. 4 shows the cross-sectional construction of a preferred embodimentof the linear array detector 104. A primary goal of this design is toprotect the electronic components forming the active detection area 130from mechanical damage, while providing a smooth structure that theautomobile 202 can drive over. The linear array detector 104 is formedfrom a solid metal base 131 having a triangular cross-section. Thealtitude 141 of this triangular cross-section is approximately 20.32 mm,while the width dimension of the base 140 is about 203.20 mm. A recessedchannel 133 in the base measures about 198.12 mm by 16.51 mm incross-section, providing a mounting location for the electroniccomponents forming the active detection area 130. A 3.175 mm thickrubber sheet 132 is affixed over the metal base 131 to exclude dirt andother contamination from the active detection area 130. As the tire 145of the automobile 202 drives over the linear array detector 104, therigidity of the tire 145 depresses the rubber sheet 132, but does notcontact the active detection area 130.

An important and key aspect of the Invention is its ability to searchautomobiles in a manner that is safe for the vehicle's occupants. Theissue of radiation safety has been specifically addressed in theANSI/HPS N43-17 standard entitled “Radiation Safety For PersonnelSecurity Screening Systems Using X-rays”, as well as a report preparedby the National Council on Radiation Protection and Measurementsentitled “Presidential Report on Radiation Protection Advice: Screeningof Humans for Security Purposes Using Ionizing Radiation ScanningSystems.” These documents clearly and explicitly put forth thatradiation exposures of less than ten microRem effective dose areacceptable for general purpose security screening. Accordingly, in apreferred embodiment of the Invention the radiation received by anydriver or passenger is less than ten microRem effective dose, or anyfuture value determined acceptable under ANSI/HPS N43-17. Under acceptedradiation protection guidelines, infrequent effective radiation doses inthe microRem range are trivial, do not need to be considered forpurposes of radiation protection, and efforts are not warranted toreduce the radiation exposure.

On the other hand, using substantially less than ten microRem perexamination results in a reduced ability to detect security threats inautomobiles. For a constant intensity of an x-ray beam, the radiationdose is proportional to the amount of time spent in the beam. Therefore,slower moving automobiles will receive a higher dose than faster movingones, for a fixed intensity beam. Prior art automobile scanning systemsinsure that the ten microRem level is met by reducing the intensity ofthe x-ray beam to a level that insures that the slowest movingautomobiles meet the limit. In turn, this results in normal and fastmoving vehicles being exposed to substantially lower radiation levels,resulting in poor detection of security threats. A preferred embodimentof the present Invention overcomes this limitation by adjusting theintensity of the x-ray beam to match the speed of the automobile.

FIG. 5 explains this aspect of the Invention. The automobile beinginspected 202 moves through the scanning apparatus 100 at some speed, inthe range of 1 mph to 20 mph. Automobile motion sensors 106 measure thisspeed. In a preferred embodiment, the automobile motion sensors 106 areone or more video cameras mounted at a location where they can view theautomobile 202 as it passes through the scanning apparatus 100. Thevideo signal 150 from the camera therefore consists of a series ofimages 160 of the automobile 202, with a second image 162 showing theautomobile displaced by a relative amount compared to a first image 161.A digital computer 170 measures this displacement in the two images 161,162 and calculates the speed of the automobile 202 by dividing thedisplacement distance by the time between images. Digital computer 170further calculates a mathematical value corresponding to the reciprocalof the speed of the automobile 202 and routes this signal 180 to aradiation intensity control 403. Radiation intensity control 403 adjuststhe intensity of the x-ray beam 114 that impinges on automobile 202according to methods known in the art, such as by changing the x-raytube current or by moving various thickness of filtration material intoand out of the beam path. The following example will explain thisoperation using a preferred embodiment of controlling the x-ray tubecurrent. As an example, when the automobile 202 is moving at 20 mph, thex-ray source 110 will be controlled to operate at a current of 2 rna.However, in this same example, an automobile 202 moving at 1 mph resultsin the x-ray source operating at 2 rna/20=0.1 rna. The total radiationexposure received by an occupant of the automobile 202 is proportionalto current divided by the speed, and will therefore be the same at allspeeds between 1 and 20 mph. In this example, 2 ma/20 mph produces thesame radiation dose as 0.1 rna/1 mph. In another preferred embodiment,placing approximately three cm of plastic or other organic matter in thebeam path will reduce the beam intensity by a factor of two. This allowsthe radiation intensity control to operate over a factor of 16 in rangeby mechanically moving a thickness between 0 and 12 cm of plastic intoand out of the beam path. This is accomplished by common mechanicaltechniques, such as a step wedge moved by a linear actuator, or arotational mechanism with a cam shaped filter material.

The effective radiation dose received by occupants of the automobile iscalculated as follows. From standard references known in the art, anx-ray tube operating at 120 KV and 2 rna produces a radiation exposureof 0.009 Roentgen per second at a distance of 2133.60 mm, theapproximate center of the automobile. The conversion between Roentgenand Rem of effective dose is approximately unity at the x-ray energiesused in the Invention. The exposure time for any location in theautomobile is 0.006 seconds. This exposure time is broken into twohalves, corresponding to the two energies needed to make the dual-energymeasurement. During both of these halves, the intensity of the beam isreduced by a factor of about ten by filtration material inserted intothe beam, as needed to shape the spectra for dual energy imaging. Thisresults in a nominal effective dose of 0.009 Roentgens/sec×0.006 sec×1Rem/Roentgen×0.1=5.4 microRem effective dose. This effective dose willincrease closer to the x-ray source; however, in no event will anyperson receive more than 10 microRem effective dose per scan.

Dual-energy x-ray imaging has long been used in security applications,for example, airport-type baggage scanners. As known in the art, anx-ray beam passing through an object is reduced in intensity accordingto:

X ₁ =X ₀ ê(−μρ t)  (1)

where X₀ is the incident x-ray intensity, X₁ is the intensity afterpassing through a material of thickness, t, and density ρ. Theparameter, μ, is the mass attenuation coefficient which depends on boththe atomic number of the material and the x-ray energy. Airport-typebaggage scanners acquire images at two separate x-ray energies,typically about 40 keV and 80 keV. This is accomplished through the useof energy sensitive detectors having a sandwich structure, such asdescribed in U.S. Pat. No. 4,626,688 issued to Barns. Two measurementsfor each pixel are therefore given by the equations:

XL ₁ =XL ₀ ê(−μ _(l) ρt)  (2)

XH ₁ =XH ₀ ê(−μ _(h) ρt)  (3)

where XL₀ and XH₀ are the intensities of the incident low andhigh-energy x-ray beams, respectively; XL₁ and XH₁ are the intensitiesof the low and high-energy x-ray beams after passing through thematerial, respectively; ρ is the density of the material; t is thethickness of the material; and μ_(l) and μ_(h) are the mass attenuationcoefficients of the material at the low and high energies, respectively.The goal of airport-type baggage scanners is to determine the atomicnumber of the material, or if one more than one element is present, aweighted or “effective” atomic number. This is accomplished bymathematically calculating a “logarithmic division,” represented here bythe variable “LD”:

LD=1n(XL ₁ /XL ₀)/1n(XH ₁ /XH ₀)=μ₁/μ_(h)  (4)

That is, the logarithmic division measures the ratio of the high to thelow-energy mass attenuation coefficients of the object being examined,which is unique for each element. For instance, carbon has a ratio ofabout 1.28; aluminum 2.8; and iron 6.2. This allows airport-type baggagescanners to identify different atomic elements and display them indifferent colors on the display monitor.

While useful for inspecting airport baggage, this technique isunworkable with the steel construction used in automobiles. First, thegoals of the examination are different. Airport baggage scanners aredesigned to present an image display to the operator, wherein the colorof each object reflects its atomic number or effective atomic number.For instance, the low atomic number elements in a water bottle orexplosive appear in one color, while the steel in a handgun appears inanother. However, it is pointless in automobile inspection to detectmetal, since it is known beforehand that large amounts of metal are usedin the construction of the vehicle. Rather, the need in automobileinspection is to detect and classify non-metal objects contained in thevehicle, in spite of the extensive overlying metal.

Second, the energy of the low-energy beam is extremely critical whenimaging through the steel present in automobiles. FIG. 6A shows the massattenuation coefficients of iron 301, which is equivalent to steel forx-ray interactions, along with water 302, which is representative oforganic materials. While the curve for water 302 is relatively flat, thecurve for iron drastically increases as the energy becomes lower. Forinstance, at 40 keV the mass attenuation coefficient of iron is 3.55cm²/gm with a density of 7.1 gm/cm³. Using equation (1), a 5 mmthickness of iron will reduce the intensity of this x-ray beam to only0.000003 of its original intensity. In typical applications about 50,000x-rays are incident in each pixel, meaning that not even a single x-rayfrom the incident beam will usually penetrate through 5 mm of iron. Incomparison, at 70 keV the mass attenuation coefficient is 0.795 cm²/gm;the penetration is 0.06; and about 3,000 x-rays penetrate through the 5mm of iron. Since automobile inspection requires penetration of at least5 mm of iron, the energy of the low-energy x-ray beam must be aboveabout 70 keV to be functional. Even though 40 keV and 70 keV may seemrelatively close in numerical value, 70 keV x-rays penetrate through 5mm of iron about 20,000 times better than 40 keV x-rays.

On the other hand, the energy of the low-energy beam cannot be higherthan about 90 keV. Discriminating between two different materials, steeland water in the present case, requires that the mass attenuationcoefficients of the two materials be significantly different at theenergy of the low-energy beam, and relatively similar at the energy ofthe high-energy beam. In FIG. 6A it can be seen that the two curves 301,302 converge as the energy becomes greater, making the energy placementof the high-energy beam relatively simple. In particular, the energy ofthe high-energy beam can be at any energy above about 90 keV. However,to achieve a difference in mass attenuation coefficients, the energy ofthe low-energy beam must be below about 90 keV. For instance, at 70 keVthe ratio of the two mass attenuation coefficients is about 4.2; at 80keV it is 3.3, and at 90 keV it is 2.8. As this ratio becomes lower, theability of the dual-energy measurement to discriminate between the twomaterials becomes less.

Taking the above two limitations together, the energy of the low-energyx-beam must be placed in the narrow window of about 70 keV to 90 keV tobe functional for the inspection of automobiles. If placed at a lowerenergy the beam will not be able to penetrate the steel of theautomobile. If placed at a higher energy insufficient dual-energyinformation can be obtained to distinguish steel from organic materials.The techniques of the prior art, such as used in airport-type baggagescanners, do not and cannot achieve placement of the low-energy beam inthis critical window.

The present Invention operates within this narrow window by usingspecific technique factors for generating the x-ray beam Specifically,the low-energy filter material must have a high k-edge energy, such asplatinum, gold, mercury, thallium, lead, bismuth, and thorium. Inaddition the x-ray tube must be operated with an electron energy that isgreater than the k-edge energy, but less than the k-edge energy plusabout 50 keV. This approximately corresponds to operating the x-ray tubebetween 100 KV and 150 KV. In a preferred embodiment, the presentinvention uses a bismuth filter with the x-ray tube operated at 120 KV.

Third, the logarithmic subtraction used in baggage scanners provide anincorrect measurement when imaging through overlying steel. As anexample, consider an explosive hidden in the trunk of an automobile.Logarithmic subtraction reports the effective atomic number of thecombination of the explosive and the overlying metal. That is, thepresence of the overlying steel corrupts the measurement. Further, thiscorruption is extreme since the attenuation of steel is far higher thanthe attenuation of organic material.

The present Invention overcomes these limitations of the prior art byproviding an apparatus and method of accurately measuring the mass oforganic objects concealed within the metal of an automobile. Operatingthe x-ray source 110 at a constant potential of approximately 120 KV,while switching filtration materials between about 6.35 mm of copper and0.762 mm of bismuth, produces the two spectra shown in FIG. 6B. Thesefiltration materials reduce the intensity of the incident x-ray beam bya factor of about ten, and in the process, optimally shape the spectrafor the particular problem of inspecting through steel. In particular,the spectrum of the high-energy beam 304 is centered at about 100 keV.The spectrum of the low-energy beam 303 is centered at about 80 keV. Akey feature of the Invention is that the bismuth filter 115 has a sharpdiscontinuity in its spectral filtration at 90.5 keV, a result of thebismuth k-edge at this energy. X-rays above 90.5 keV are highlyattenuated, essentially removing them from the spectrum 303. As can beseen in FIG. 6B, this effect generates a low-energy beam 303 that isideally placed in the 70 keV to 90 keV window that is critical forautomobile inspection.

Another important aspect of the present Invention is the use of“logarithmic subtraction”, as opposed to the “logarithmic division”previously described and used in prior art security systems. Themathematics of logarithmic subtraction are explained as follows, usingthe simplified example of mono-energetic x-ray beams. The followingequations represent the high and low-energy beams passing through anobject composed of both iron and water:

XH ₁ =XH ₀ ê(−μ _(hi)ρ_(i) t _(i))ê(−μ _(hw)ρ_(w) t _(w))  (5)

XL ₁ =XL ₀ ê(−μ _(li)ρ_(i) t _(i))ê(−μ _(lw)ρ_(w) t _(w))  (6)

where XH₀ and XL₀ are the incident intensities of the high andlow-energy x-ray beams, respectively; XH₁ and XL₁ are the intensities ofthe x-ray beams after passing through the object; μ_(hi), μ_(hw),μ_(li), and μ_(lw) are the mass attenuation coefficients of iron andwater at the high and low x-ray energies, as denoted by the subscripts;and ρ_(i) and ρ_(w) are the densities of iron and water, respectively.These equations can be simplified by expressing the signal for each beamas an attenuation, expressed in dB, as the beam passes through theobject:

H=−20 log(XH ₁ /XH ₀)  (7)

L=−20 log(XL ₁ /XL ₀)  (8)

where H and L are the attenuation of the high and low-energy beamsexpressed in dB, respectively; and the other variables are as previouslydefined. By combining equations (5), (6), (7) and (8), and reducing byelementary algebra, the following equations are found:

t _(w) =k[H−rL]  (9)

where:

k=μ _(il)/(ρ_(w)(μ_(wh)μ_(il)−μ_(ih)μ_(wl)))  (10)

r=μ _(ih)/μ_(il)  (11)

In (10) and (11) the parameters k and r are shown to simply be constantsthat depend on the fixed mass attenuation coefficients and density.Equation (9) shows the goal of the logarithmic subtraction; thethickness of the water can be calculated from the measured values andknown constants, regardless of the thickness of metal present. That is,applying equation (9) to each pixel creates an electronic image wherethe value of each pixel is the thickness of the water, with thecorrupting effect of overlying metal completely removed.

The above mathematical analysis assumes that mono-energetic x-ray beamsare used, making all of the mass attenuation coefficients a fixed value.However, conventional x-ray sources produce polyenergetic x-ray beamsthat change their energy spectra as they pass through material, makingthe value of the mass attenuation coefficients a variable of theequations. This effect can be seen in FIG. 7A, showing the attenuationof the low and high-energy beams of the Inventive system 313, 314, asthe beams pass through iron. Corresponding curves for water 323, 324 areshown in FIG. 7B. Two important facts can be learned from the shape ofthese curves. First, the attenuation in iron of both the high andlow-energy beams 313, 314 are nonlinear in this graph, that is, they arenot straight lines. This means that the effect of overlying steel in theInventive system cannot be accounted for by a simple constant ofproportionality, such as “r” in equations (9) and (11). A more intricatecalibration method is required. However, the attenuation through wateris very linear in this graph for both beams 323, 324. Accordingly, asimple constant of proportionality can adequately account for the waterthickness measurement in the Inventive system. These general conceptsare embodied in the following calibration procedure of a preferredembodiment of the Invention.

1. Correct for DC Offset

With the x-ray beam not energized, each detector element in the activedetection area 130 will output a slightly different level of electronicsignal as a result in variations in the electronic components. In apreferred embodiment, the value of the signal from each detector elementis measured and stored with the x-ray beam turned off. All futuremeasurements from the Invention are then modified by subtracting thestored values from the measured value to correct for the detector DCoffset.

2. Convert to Logarithmic Domain

With no object in the fan beam 114, the value of the signal from eachdetector element is measured and stored with the low-energy beam turnon, referred to hereafter as XL₀. In a similar fashion, the value of thesignal from each detector element is measured and stored with thehigh-energy beam turn on, referred to hereafter as XH₀. All futuremeasurements from the Invention are then converted into a logarithmicattenuation. In the previous discussion for equations (7) and (8) theseattenuations were expressed separately for the high and low-energybeams. However, in a preferred embodiment the calibration procedure ismore easily carried out by calculating the sum and differenceattenuations, as follows:

XSUM=−20 log((XL ₁ +XH ₁)/(XL ₀ +XH ₀))  (12)

XDIF=−20 log((XL ₁ −XH ₀)/(XH ₁ −XL ₀))  (13)

where XH₁ is a measured value of the signal from each detector elementfor the high-energy beam; XL₁ is a measured value of the signal fromeach detector element for the low-energy beam; XSUM is the value of thesignal from each detector element for the sum of the high and low-energybeams expressed as an attenuation in dB; and XDIF is the value of thesignal from each detector element for the difference between the highand low-energy beams expressed as an attenuation in dB. In other words,XSUM is the measured attenuation, expressed in dB, of the combination ofboth the high and the low-energy beams. Likewise, XDIF is the differencebetween the measured attenuations of the high and the low-energy beams,also expressed in dB.

3. Determine Calibration Data

As explained previously, the attenuation of the x-ray beams through ironcannot be expressed by a single constant. This step determines themultipoint calibration data needed to correct for this effect. In thepreferred embodiment this is accomplished by acquiring images of fourdifferent thickness of iron sheets, ⅛″ (3.175 mm), ¼″ (6.35 mm), ⅜″(9.525 mm) and ½″ (12.70 mm). The value of XSUM and XDIF is measured foreach thickness, taken as the average of the multitude of pixels thatcorrespond to the object in each respective image. FIG. 8 shows thetypical values of XSUM plotted against the values of XDIF for ironthickness of ⅛″ (3.175 mm), ¼″ (6.35 mm), ⅜″ (9.525 mm) and ½″ (12.70mm); 332, 333, 334, 335, respectively. Also shown is the known zerocondition 331 where XSUM=0 and XDIF=0. These five points 331-335 definea curve 330 relating the value of XSUM to XDIF for all thickness of ironin the useful range of the invention. In the calibration procedure ofthe preferred embodiment, this curve 330 is held in a computer arraywith indexes 0 to 1400, corresponding to the value of XDIF being 0 to 14dB. The value of the measured points 331-335 are inserted directly intothis array, and the points between determined by a curve fit. This arraydefines the calibration needed to correct for various thickness of ironin the operational images, and therefore will be referred to as the“Iron Correction Factor array”, represented by the notation, ICF[ ].That is, the ICF[ ] array provides a lookup table that converts anymeasured value of XDIF into the corresponding value of XSUM that wouldoccur if only iron were being measured.

As also previously explained, the attenuation of the x-ray beams throughwater can essentially be represented by a single constant. Thecalibration procedure of a preferred embodiment determines this constantby taking an image of a 101.60 mm thick container of water affixed to a⅛″ (3.175 mm) thick sheet of iron. The values of XSUM and XDIF for thiscalibration phantom are measured from the acquired image. The value ofthe calibration constant, k, is then calculated as:

k=101.60 mm/(XSUM−ICF[XDIF])  (14)

The nominal value of k is 9.0932 mm per dB. The value of k, plus thevalues in the iron correction factor array, ICF[ ], form the data neededto calibrate the operation of the Invention.

4. Measure the Thickness of Organic Material in an Acquired Image

Each pixel in the image of a scanned automobile, represented by a valueof XSUM and XDIF, is converted into the thickness of water correspondingto that pixel by:

t _(w) =k(XSUM−ICF[XDIF])  (15)

This method of calibrating the dual-energy information achieves anaccuracy of +/−3.81 mm, over a 0 to 304.80 mm range of water thickness,and 0 to ⅜″ (9.525 mm) range of iron thickness.

A preferred embodiment of the method of inspecting a vehicle will now bedescribed and further explained in FIG. 9. The first step is to acquirea dual-energy x-ray image of the vehicle 191. This step comprisesgenerating x-rays of at least two different energies, directing thex-rays through the automobile, and detecting the x-ray that exit theautomobile. The x-ray image acquired in this step resides as digitalcomputer data, consisting of a plurality of pixels, with each pixelconsisting of measured data for the at least two different energies.

The second step is to calculate a steel suppressed image 192. This stepis carried out using the previously described logarithmic subtraction.This step may be carried out analytically, as described in equations(5)-(11). Alternatively, it may be carried out through the use ofcalibrated lookup tables, such as discussed in conjunction with FIG. 8and the portion of equation (15) denoted by: “[XSUM−ICF(XDIF)]”. Thesteel suppressed image calculated in this step resided as digitalcomputer data, consisting of a plurality of pixels, with the value ofeach pixel being immune to the effect of steel in the vehicle.

The third step is to calibrate the steel suppressed image 193. In thepreferred embodiment this step is carried out multiplying each pixel inthe steel suppressed image by a calibration factor, referred to as “k”,in equations (14) and (15). At the completion of this step, each pixelin the calibrated steel suppressed image is a direct measure of thethickness of water, organic, or other non-steel objects present in thevehicle at the location corresponding to the pixel.

The fourth step is to determine object boundaries 194. The goal of thisstep is to identify groups of pixels in the calibrated steel suppressedimage that correspond to each of the water, organic, or other non-steelobjects present in the vehicle. In the preferred embodiment thiscomprises thresholding the image to eliminate all regions that have ameasured thickness of less than about 25.40 mm. After thresholding,groups of pixels are identified in the image that are connected. In apreferred embodiment, computer algorithms known in the art as “blobanalysis” are used to further refine the object boundaries of eachconnected group of pixels to most closely correspond to actual objectscontained in the vehicle. At the completion of this step, the pixelsthat correspond to each object in the vehicle are identified and resideas digital data in a computer.

The fifth step is to calculate the mass of each object from itsboundaries and the calibrated image 195. The value of each pixel in thecalibrated image, calculated in step 3, is the measured thickness of thecorresponding object at that pixel location. The boundaries of eachobject determined in step 4 provide the projected area of the object. Ina preferred embodiment this step 195 comprises calculating the mass ofeach object by summing the values of all pixels contained within theobject boundaries, and multiplying by the assumed density of the object.In a preferred embodiment all organic material is assumed to have thecharacteristics of water, with the density of 1 gm/cm³. The result ofthis step is a list of masses associated with each object in thevehicle, held as digital data in a computer. The assumption that allorganic material has the characteristics of water results in an errorthat is not significant for the purposes and goals of the Invention.

The sixth step is the decision: “Does the mass of any object exceed athreshold?” 196. This step comprises a computer comparing each of theobject masses calculated in step five with a predetermined massthreshold. This mass threshold depends on the application where theInvention is being deployed. For example, a border checkpoint may setthe threshold to 22.680 kg to detect the presence of persons hidden inthe vehicle. In comparison, the entrance to the underground parkingfacility of a skyscraper may set the threshold to 226.80 kg to detectcar bombs.

The seventh step is to trigger an alarm 197, if the answer determined instep six is “yes.” In the preferred embodiment this alarm consists of anaudible sound in the area where a security officer is stationed, alongwith a visual presentation on a computer monitor indicating the locationin the scanned image where the triggering object is located.

The above specific descriptions and embodiments have been made toexplain the Invention and those skilled in the art will immediatelyrecognize that other embodiments and modifications are within the scopeof the Invention. For instance: The x-ray source may use a fixed orrotating target; operate with other combinations and variations oftechnique factors; be cooled by air, water, or oil; and otherembodiments that are known in the art. The x-ray detector may compriseother scintillation crystals or screens; use photomultiplier tubes,other electron multiplication devices, or other light detectiontechnologies known in the art; be mounted in other configurations toprevent damage to the components; or use other x-ray detectors known inthe art. Switching the beam filtration may be done by a linear actuator,flat wheel, cylinder, or other mechanical assembly. Modulation of theintensity of the x-ray beam may be accomplished by varying the x-raytube beam current, KV, or mechanically placing filters in the beam.Computer calculations may be carried out in alternative ways known inthe art to achieve the same goals. The selection of beam filtrationmaterials may extend to elements and compounds that have similarcharacteristics to those stated for the preferred embodiment of copperand bismuth. For instance, copper may be replaced by iron, nickel, zinc,silver, molybdenum, or tin, or combinations of these elements. Bismuthmay be replaced by, for instance, platinum, gold, mercury, thallium,lead, bismuth, or thorium, or combinations of these elements.Calibration of the Invention may be accomplished by phantoms constructedof steel instead of iron, since iron and steel are essentiallyequivalent in x-ray characteristics. Further, calibration of theInvention may be made in thicknesses of organic materials other thanwater, such as plastics or explosives. Variations in the geometric sizeof the Invention may be made, such as making it large enough to examinetrucks and buses. Although particular embodiments of the Invention havebeen described in detail for the purpose of illustration, various othermodifications may be made without departing from the spirit and scope ofthe Invention.

1. An apparatus for imaging an organic object contained within a metalenclosure, comprising: an x-ray source for producing an x-ray beampassing through said metal enclosure; a first beam filter for creating alow-energy x-ray spectrum; said first beam filter comprising an atomicelement having a high k-edge energy; a second beam filter for creating ahigh-energy x-ray spectrum; a filter exchanger for alternatelypositioning said first beam filter and said second beam filter withinsaid x-ray beam; an x-ray detector for measuring an intensity of saidx-ray beam exiting said metal enclosure; a computer for converting saiddigitally represented signal into a steel suppressed digital image ofsaid automobile, the computer being programmed to: calibrate thedigitally represented signal to correct for multiple thicknesses of ironin the automobile; calibrate the digitally represented signal to correctfor organic materials in the automobile; whereby the image of saidorganic object is isolated from the interference of said metalenclosure.
 2. The apparatus of claim 1, wherein said metal enclosure isan automobile.
 3. The apparatus of claim 1, wherein said x-ray sourcecomprises an x-ray tube operating with an electron energy greater thanthe k-edge energy of said atomic element.
 4. The apparatus of claim 3,wherein said x-ray tube further operates with an electron energy lessthan the sum of the k-edge energy and 50 keV.
 5. The apparatus of claim1, wherein said atomic element is selected from the group consisting ofplatinum, gold, mercury, thallium, lead, bismuth, and thorium.
 6. Theapparatus of claim 4, wherein said atomic element is selected from thegroup consisting of platinum, gold, mercury, thallium, lead, bismuth,and thorium.
 7. The apparatus of claim 6, wherein the atomic compositionof said second beam filter is selected from the group consisting ofiron, nickel, copper, zinc, silver, molybdenum, and tin.
 8. Theapparatus of claim 1, wherein said computer calculates said metalsuppressed image by means of a logarithmic subtraction.
 9. The apparatusof claim 1, wherein said filter exchanger comprises a rotating assembly.10. The apparatus of claim 7, wherein said filter exchanger comprises arotating assembly.
 11. The apparatus of claim 1, wherein said x-raydetector comprises a linear array of scintillation crystals affixed tophotodiodes.
 12. An apparatus for searching an automobile for securitythreats, comprising: x-ray generating means for penetrating saidautomobile with x-ray radiation; switching filter means for changingsaid x-ray radiation between a low-energy spectrum and a high-energyspectrum, wherein said low-energy spectrum is substantially determinedby the k-edge energy of a high atomic number element; x-ray detectormeans for converting the x-ray radiation that penetrates said automobileinto a digitally represented signal; a computer for converting saiddigitally represented signal into a steel suppressed digital image ofsaid automobile, the computer being programmed to: calibrate thedigitally represented signal to correct for multiple thicknesses of ironin the automobile; calibrate the digitally represented signal to correctfor organic materials in the automobile.
 13. The apparatus of claim 12,wherein said high atomic number element is selected from the groupconsisting of platinum, gold, mercury, thallium, lead, bismuth, andthorium.
 14. The apparatus of claim 13, wherein said x-ray generatingmeans comprises an x-ray tube operating between 100 KV and 150 KV. 15.The apparatus of claim 12, wherein said switching filter means comprisesrotating filter means, said rotating filter means comprising alow-energy filter material and a high-energy filter material.
 16. Theapparatus of claim 15, wherein said low-energy filter material isselected from the group consisting of platinum, gold, mercury, thallium,lead, bismuth, and thorium.
 17. The apparatus of claim 16, wherein saidhigh-energy filter material is selected from the group consisting ofiron, nickel, copper, zinc, silver, molybdenum, and tin.
 18. Theapparatus of claim 12, wherein said computer means calculates the steelsuppressed image by a logarithmic subtraction.
 19. The apparatus ofclaim 16, wherein said computer means calculates the steel suppressedimage by a logarithmic subtraction.